A conventional Bragg grating comprises an optical fiber in which the index of refraction undergoes periodic perturbations along its length. The perturbations may be equally spaced in the case of an unchirped grating, or may be unequally spaced in the case of a chirped grating. The grating reflects light over a given waveband centered around a wavelength equal to twice the spacing between successive perturbations. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings are typically employed in a variety of application including filtering, stabilization of semiconductor lasers, reflection of fiber amplifier pump energy, and compensation for fiber dispersion.
Both the refractive index of the grating and the distance between successive perturbations are temperature dependent. As a result, the reflected waveband is also temperature dependent. In many cases, however, it is desirable to provide a stabilized reflection band that is temperature independent. U.S. Pat. No. 5,042,898 (Morey et al.) discloses a temperature-independent Bragg grating in which wavelength changes resulting from changes in strain are used to compensate for wavelength changes resulting from variations in the temperature of the grating. For example, a constant wavelength of reflected light may be maintained during a drop in temperature by increasing the longitudinal strain on the fiber. In this reference, a portion of the optical fiber containing the grating is sectioned off by securing the optical fiber at each side of the grating to separate metallic compensating members arranged for longitudinal movement relative to one another. By mechanically adjusting the compensating members longitudinal relative to each other to thereby vary the distance between them, there is imposed on the optical grating a longitudinal strain of a magnitude that varies to balance out wavelength variations resulting from changes in the temperature of the grating. This known temperature compensating package however, is cumbersome and expensive to manufacture.
In some cases, the precise value of the center wavelength of the reflection band is not very critical. This is often the case with gratings that have a wide bandwidth, e.g., greater than 1 nm, such as very long dispersion compensating gratings. It is important, however, that the entire length of such a grating be maintained at the same temperature. Otherwise, portions of the grating at different temperatures will reflect different wavelengths, distorting rather than merely shifting the reflection band.
Accordingly, it would be desirable to provide a package for a Bragg grating that is relatively simple and inexpensive to manufacture, and which maintains the entire Bragg grating at a uniform temperature.